Method of manufacturing stretchable circuit assemblies

ABSTRACT

A stretchable circuit assembly includes first and second printed circuit boards, discrete conductive wires or flexible circuits including ends connected to the first and second printed circuit boards, and a stretchable interconnect in which the discrete conductive wires or flexible circuits and a portion of the first and second printed circuit boards are embedded. Main surfaces of the flexible circuits are perpendicular or substantially perpendicular to main surfaces of the stretchable interconnect. A method of making a stretchable circuit assembly includes the steps of providing electrical interconnects, a first printed circuit board, and a second printed circuit board, shaping the electrical interconnects to have an oscillating configuration, and forming a stretchable interconnect such that the electrical interconnects, a portion of the first printed circuit board, and a portion of the second printed circuit board are embedded within the stretchable interconnect.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stretchable circuits. Morespecifically, the present invention relates to stretchable circuits thatare polymer based and include conductive wires or flexible circuitsembedded in the stretchable circuits.

2. Description of the Related Art

Known flexible circuits are implemented using flexible printed circuits.While flexible circuits can bend, they cannot stretch. To offerelectrical connections that can elongate, flexible circuits are foldedso that they can slide. Such known flexible circuits can slide back andforth with a specified bend gap and are intended to last for more than200,000 sliding cycles. As the bend gap of the flexible circuitdecreases, the number of cycles before failure occurs reducesexponentially.

U.S. Pat. No. 7,337,012 B2 teaches a stretchable circuit including astretchable polymer body with micro-channels that are filled withconductive material. U.S. Pat. No. 7,337,012 B2 does not discuss theelectrical terminals necessary to connect the stretchable circuit toother components. U.S. Pat. No. 7,337,012 B2 uses a conductor that is inliquid or paste form. The micro-channels are created in the substrate,and then the conductor is formed by forcing the liquid or paste into themicro-channels. Thus, the conductor takes the shape of themicro-channels. The liquids and pastes used in U.S. Pat. No. 7,337,012B2 have a much higher bulk resistivity, in the range of three to tentimes, than the bulk resistivity of copper wire. A higher resistanceproduces a lower performing circuit, which will not be suitable for manyelectronic applications.

U.S. Patent Application Publication No. 2009/0317639 A1 teachesconventional stretchable circuits using flexible circuits. Thestretchable circuits are formed by laser cutting or die cutting theflexible circuits to form patterns in the flexible circuits. Portions ofthe flexible circuit are then removed to define stretchable conductiveelements. This conventional stretchable circuit is then embedded in apolymer. However, in this conventional stretchable circuit, the flexiblecircuit and the conductive patterns are on the same plane, which causesthe thickness of the polymer to be greater than optimal. Further, thisstretchable circuit does not use conductive wires.

International Patent Application No. WO 2010/086034 A1 also teaches aconventional stretchable circuit. Portions of the stretchable circuithave different stiffnesses, which allows the stretchable circuit tostretch. To form the stretchable circuit, flexible circuits are lasercut, and the portions of the flexible circuits that are not needed areremoved. The resulting stretchable circuit is then embedded in polymer.The conductive patterns are on the same plane as the body of thecircuit, which causes the thickness of the polymer to be greater thanoptimal.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a stretchable circuit assembly includingconductive wires or flexible circuits embedded within a stretchableinterconnect.

In a first preferred embodiment of the present invention, a stretchablecircuit assembly includes first and second printed circuit boards,discrete conductive wires including ends connected to the first andsecond printed circuit boards, and a stretchable interconnect in whichthe discrete conductive wires and a portion of the first and secondprinted circuit boards are embedded.

The stretchable circuit assembly preferably further includes a strainrelief wire embedded in the stretchable interconnect and arranged toprevent the stretchable interconnect from being stretched such that thediscrete conductive wires are damaged. The ends of the discreteconductive wires are preferably soldered to the first and second printedcircuit boards. The discrete conductive wires preferably have anoscillating configuration. The discrete conductive wires preferablyinclude semi-circular shaped portions connected by linear portions.

In a second preferred embodiment of the present invention, a stretchablecircuit assembly includes first and second printed circuit boards,flexible circuits including ends connected to the first and secondprinted circuit boards, and a stretchable interconnect in which theflexible circuits and a portion of the first and second printed circuitboards are embedded. Main surfaces of the flexible circuits areperpendicular or substantially perpendicular to main surfaces of thestretchable interconnect.

The stretchable circuit assembly preferably further includes a strainrelief circuit embedded in the stretchable interconnect and arranged toprevent the stretchable interconnect from being stretched such that theflexible circuits are damaged. Ends of the flexible circuits arepreferably soldered to the first and second printed circuit boards. Theflexible circuits preferably have an oscillating configuration. Theflexible circuits preferably include semi-circular shaped portionsconnected by linear portions.

In a third preferred embodiment of the present invention, a method ofmaking a stretchable circuit assembly includes the steps of providingelectrical interconnects, a first printed circuit board, and a secondprinted circuit board; shaping the electrical interconnects to have anoscillating configuration; and forming a stretchable interconnect suchthat the electrical interconnects, a portion of the first printedcircuit board, and a portion of the second printed circuit board areembedded within the stretchable interconnect.

The electrical interconnects preferably include one of conductive wiresand flexible circuits. The step of forming a stretchable interconnect ispreferably performed by injection molding. The step of forming astretchable interconnect preferably uses a polymer. The method of makinga stretchable circuit assembly preferably further includes the step ofattaching ends of the electrical interconnects to the first printedcircuit board and the second printed circuit board.

The above and other features, elements, characteristics and advantagesof the present invention will become more apparent from the followingdetailed description of preferred embodiments of the present inventionwith reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top plan view of a stretchable circuit assembly accordingto a first preferred embodiment of the present invention.

FIG. 1B is a bottom plan view of a stretchable circuit assemblyaccording to a first preferred embodiment of the present invention.

FIG. 1C is a top plan view of a stretchable circuit assembly showing theconductive wires according to a first preferred embodiment of thepresent invention.

FIG. 2A is a perspective view of a stretchable circuit assemblyaccording to a first preferred embodiment of the present invention.

FIG. 2B is a perspective view of a stretchable circuit assembly showingthe conductive wires according to a first preferred embodiment of thepresent invention.

FIG. 3A is a top plan view of a stretchable circuit assembly accordingto a second preferred embodiment of the present invention.

FIG. 3B is a bottom plan view of a stretchable circuit assemblyaccording to a second preferred embodiment of the present invention.

FIG. 3C is a top plan view of a stretchable circuit assembly showing theflexible circuits according to a second preferred embodiment of thepresent invention.

FIG. 4A is a perspective view of a stretchable circuit assemblyaccording to a second preferred embodiment of the present invention.

FIG. 4B is a perspective view of a stretchable circuit assembly showingthe flexible circuits according to a second preferred embodiment of thepresent invention.

FIG. 4C is a close-up view of a flexible circuit according to a secondpreferred embodiment of the present invention.

FIGS. 5A and 5B show an injection mold used in a method of manufacturinga stretchable circuit assembly according to a third preferred embodimentof the present invention.

FIG. 6 shows a flexible printed circuit used in a method ofmanufacturing a stretchable circuit assembly according to a thirdpreferred embodiment of the present invention.

FIGS. 7A-7C show top plan views of a mold core used in a method ofmanufacturing a stretchable circuit assembly according to a thirdpreferred embodiment of the present invention.

FIGS. 8A-8C shows side views of a mold core used in a method ofmanufacturing a stretchable circuit assembly according to a thirdpreferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A-2B show a stretchable circuit assembly 10 with conductive wires13 according to a first preferred embodiment of the present invention.FIGS. 3A-4B show a stretchable circuit assembly 20 with flexiblecircuits 23 according to a second preferred embodiment of the presentinvention. FIGS. 5A-8B show a method of manufacturing a stretchablecircuit assembly according to a third preferred embodiment of thepresent invention.

Stretchable Circuit Assembly with Conductive Wires

FIGS. 1A-2B show a stretchable circuit assembly 10 according to a firstpreferred embodiment of the present invention. The stretchable circuitassembly 10 includes two printed circuit boards 11 a, 11 b, astretchable interconnect 12 connected between the two printed circuitboards 11 a, 11 b, and conductive wires 13 connected to the two printedcircuit boards 11 a, 11 b and embedded within the stretchableinterconnect 12.

The stretchable interconnect 12 is preferably formed such that theconductive wires 13 and the two printed circuit boards 11 a,11 b areembedded within the stretchable interconnect 12. The stretchableinterconnect 12 is made of a material that is stretchable so that thedistance between the two printed circuit boards 11 a, 11 b can beincreased by stretching the stretchable interconnect 12. The stretchableinterconnect 12 is preferably a polymer such as polydimethylsiloxane(PDMS); however, other suitable stretchable materials, such as urethane,polyurethane elastomers, hydrocarbon rubber/elastomers, and polyetherblock amides (PEBA), can also be used.

Although only one stretchable interconnect 12 is shown in FIGS. 1A-2Bconnecting the two printed circuit boards 11 a, 11 b, it is possible tohave more than one stretchable interconnect connecting two printedcircuit boards. Although only two printed circuit boards 11 a, 11 b areshown in FIGS. 1A-2B, it is also possible to connect more than twoprinted circuit boards. For example, two stretchable interconnects couldconnect three printed circuit boards together in a chain, with two sidesof a middle printed circuit board connected to one of the stretchableinterconnects that is connected to a printed circuit board at the end ofthe chain. Another example is for two stretchable interconnects to beconnected at one end to the same side of the same printed circuit boardand at the other end to separate printed circuit boards. It is alsopossible to use an adhesive to more securely attach the stretchableinterconnect 12 to the two printed circuit boards 11 a, 11 b. Anysuitable adhesive can be used.

The conductive wires 13 are preferably attached to the printed circuitboards 11 a, 11 b by using solder 14. However, the conductive wires 13could be attached to the printed circuit boards 11 a, 11 b using anysuitable method. For example, the conductive wires 13 could be attachedto the printed circuit boards 11 a, 11 b by bonding with electricallyconductive epoxy adhesive, clamping, pressure fittings, and crimping.The conductive wires 13 can be made of any suitable conductive material.The conductive wires 13 are preferably made of a conductive metal suchas copper, silver, gold, or aluminum. The conductive wires 13 can becoated or uncoated. If the conductive wires 13 are not coated, then thestretchable interconnect 12 acts as a dielectric between the individualconductive wires 13 to prevent shorting between adjacent conductivewires 13. The conductive wires 13 are preferably discrete wires that arefabricated in bulk for commercial sale. That is, the conductive wires 13are shaped and formed before being embedded in the stretchableinterconnect 12 as compared to the conductors formed in U.S. Pat. No.7,337,012 B2 by forcing a liquid or paste into micro-channels in asubstrate.

Although not shown in the figures, it is possible to have more than oneconductive line 13 that have the same shape when viewed in plan view butthat are vertically separated from each other when viewed in across-sectional view. For example, two conductive lines 13 could beused, with one of the conductive lines 13 soldered to the top of theprinted circuit boards 11 a, 11 b and with the other of the conductivelines 13 soldered to the bottom of the printed circuit boards 11 a, 11b. Of course, having more than one conductive line 13 spaced apart fromeach other when viewed in cross-section requires that the stretchableinterconnect 12 be thicker than when only one conductive line 13 isused.

In FIGS. 1C and 2B, the stretchable interconnect 12 is shown assee-through so that the shape of the conductive wires 13 can be seen.The conductive wires 13 preferably have an oscillating or meanderingshape as shown in FIGS. 1C and 2B so that shape of the conductive wires13 changes as the stretchable circuit assembly 10 is stretched in thedirection between the printed circuit boards 11 a, 11 b. For example,the conductive wires 13 can have semi-circular portions connected withlinear portions as shown in FIGS. 1C and 2B. Instead of semi-circularportions, the conductive wires 13 could have triangular portions orother similar shaped portions. It is also possible to not include linearportions in the conductive wires 13 and to have the semi-circularportions, or other suitable shape connected to each other. Otherconfigurations are also possible as long as the conductive wires 13 arenot damaged and the signal integrity of the signals transmitted throughthe conductive wires 13 is maintained when the stretchable interconnect12 is stretched. The stretch range of the stretchable circuit assembly10 is typically between 0% and about 125% of the length of thestretchable interconnect 12.

One or more of the conductive wires 13 can be replaced by strain reliefwires 15. The strain relief wires 15 prevent the stretchableinterconnect 12 from being over stretched, which protects the conductivewires 13 from being damaged. Preferably, as shown in FIGS. 1A-2B, thetop and bottom conductive wires 13 are replaced by strain relief wires15. However, no strain relief wires 15 can be used; one strain reliefwire 15, e.g., in the middle of the stretchable interconnect 12, can beused; and more than two relief wires 15, e.g., alternating with theconductive wires 13, can be used.

The strain relief wires 15 can be made of any suitable material,including, for example, a metal or carbon fiber. The strain relief wires15 preferably have the same shape as the conductive wires 13 as shown inFIGS. 1A-2B; however, it also possible for the strain relief wires 15 tohave a shape different from the conductive wires 13.

Any suitable printed circuit board can be used for the printed circuitboards 11 a, 11 b. Although not shown in FIGS. 1A-2B, the printedcircuit boards 11 a, 11 b can include active or passive components forprocessing and/or modifying the signals transmitted through thestretchable circuit assembly 10. Although not shown in FIGS. 1A-2B, theprinted circuit boards 11 a, 11 b preferably include an electricalconnector for connecting the stretchable circuit assembly 10 toelectrical devices with a corresponding electrical connector. Theprinted circuit boards 11 a, 11 b can be attached to an electronicdevice via surface mounted connectors such as board-to-board, clamping,pressure fittings, or spring pins, or can be embedded within a secondaryprinted circuit board, drilled, and copper plated to create a viainterconnect.

Stretchable Circuit Assembly with Flexible Circuits

FIGS. 3A-4B show a stretchable circuit assembly 20 according to a secondpreferred embodiment of the present invention. The stretchable circuitassembly 20 includes two printed circuit boards 21 a, 21 b, astretchable interconnect 22 connected between the two printed circuitboards 21 a, 21 b, and flexible circuits 23 connected to the two printedcircuit boards 21 a, 21 b and embedded within the stretchableinterconnect 22.

The stretchable interconnect 22 is preferably formed such that theflexible circuits 23 and the two printed circuit boards 21 a, 21 b areembedded within the stretchable interconnect 22. The flexible circuits23 are preferably embedded within the stretchable interconnect 22 suchthat the main surfaces of the flexible circuits 23 are perpendicular orsubstantially perpendicular to the main surfaces of the stretchableinterconnect 22. By arranging the main surface of the flexible circuits23 perpendicular or substantially perpendicular to the main surface ofthe stretchable interconnect 22, it is possible to increase the lengthof the flexible circuits 23 without increasing the thickness of thestretchable interconnect 22. FIG. 4B shows axes for the thickness T₁,width W₁, and length L₁ of the stretchable interconnect 22 and for thethickness T₂, width W₂, and length L₂ of the flexible circuits 23. Thethicknesses T₁, T₂ are in the smallest dimensions of the stretchableinterconnect 22 and flexible circuits 23, respectively. Because thethicknesses T₁, T₂ are perpendicular or substantially perpendicular(i.e., the thickness T₁ of the stretchable interconnect 22 and the widthW₂ of the flexible circuits 23 are parallel or substantially parallel),the width W₂ of the flexible circuit 23 and the thickness T₁ of thestretchable interconnect 22 can be substantially the same. Because ofthe oscillating shape of the flexible circuits 23, when the length ofthe flexible circuits 23 is increased, it might be necessary to increasethe width of the stretchable interconnect 22. By increasing the lengthof the flexible circuits 23, the stretch range of the stretchablecircuit assembly 20 is increased.

The stretchable interconnect 22 is made of a material that isstretchable so that the distance between the two printed circuit boards21 a, 21 b can be increased by stretching the stretchable interconnect22. The stretchable interconnect 22 is preferably a polymer such aspolydimethylsiloxane (PDMS); however, other suitable stretchablematerials, such as urethane, polyurethane elastomers, hydrocarbonrubber/elastomers, and polyether block amides (PEBA), can also be used.

Although only one stretchable interconnect 22 is shown in FIGS. 3A-4Bconnecting the two printed circuit boards 21 a, 21 b, it is possible tohave more than one stretchable interconnect connecting two printedcircuit boards. Although only two printed circuit boards 21 a, 21 b areshown in FIGS. 3A-4B, it is also possible to connect more than twoprinted circuit boards. For example, two stretchable interconnects couldconnect three printed circuit boards together in a chain, with two sidesof a middle printed circuit board connected to one of the stretchableinterconnects that is connected to a printed circuit board at the end ofthe chain. Another example is for two stretchable interconnects to beconnected at one end to the same side of the same printed circuit boardand at the other end to separate printed circuit boards. It is alsopossible to use an adhesive to more securely attach the stretchableinterconnect 22 to the two printed circuit boards 21 a, 21 b. Anysuitable adhesive can be used.

The flexible circuits 23 are preferably attached to the printed circuitboards 21 a, 21 b by using solder. However, the flexible circuits 23could be attached to the printed circuit boards 21 a, 21 b using anysuitable method. For example, the flexible circuits 23 could be attachedto the printed circuit boards 21 a, 21 b by bonding with electricallyconductive epoxy adhesive, clamping, pressure fittings, and crimping.The ends of the flexible circuits 23 preferably have an L- or a reverseL-shape. The ends of the flexible circuits 23 are preferably insertedthrough holes 26 in the printed circuit boards 21 a, 21 b. After theends of the flexible circuits 23 are inserted through holes 26 in theprinted circuit boards 21 a, 21 b, the ends of the flexible circuits 23are soldered to the printed circuit boards 21 a, 21 b.

Typically, the flexible circuits 23 include a flexible plastic substratewith one or more conductive lines 27 for transmitting electronicsignals. FIG. 4C is close-up view of one of the flexible circuits 23with conductive lines 27. The flexible plastic substrate can be apolyimide, a polyether ether ketone (PEEK), a transparent conductivepolyester, or any other suitable flexible material. The conductive lines27 can be made of any suitable electrically conducting material. Theflexible circuits 23 can include passive and/or active components thatprocess and/or modify the signals transmitted through the stretchablecircuit assembly 10. Although not shown in the figures, it is possibleto have two or more flexible circuits 23 that have the same shape whenviewed in plan view but that are vertically separated from each otherwhen viewed in a cross-sectional view. For example, two flexiblecircuits 23 could be used, with one of the flexible circuits 23 solderedto the top of the printed circuit boards 21 a, 21 b and with the otherof the flexible circuits 23 soldered to the bottom of the printedcircuit boards 21 a, 21 b. Of course, having more than one flexiblecircuit 23 spaced apart from each other when viewed in cross-sectionrequires that the stretchable interconnect 22 be thicker than when onlyone flexible circuit 23 is used.

In FIGS. 3C and 4B, the stretchable interconnect 22 and the printedcircuit boards 21 a, 21 b are shown as see-through so that the shape ofthe flexible circuits 23 can be seen. The flexible circuits 23preferably have an oscillating shape as shown in FIGS. 3C and 4B so thatshape of the flexible circuits 23 changes as the stretchable circuitassembly 20 is stretched in the direction between the printed circuitboards 21 a, 21 b. For example, the flexible circuits 23 can havesemi-circular portions connected with linear portions as shown in FIGS.3C and 4B. Instead of semi-circular portions, the flexible circuits 23could have triangular portions or other similar shaped portions. It isalso possible to not include linear portions in the flexible circuit 23and to have the semi-circular portions, or other suitable shape,connected to each other. Other configurations are also possible as longas the flexible circuits 23 are not damaged and the signal integrity ofthe signals transmitted through the flexible circuits 23 is maintainedwhen the stretchable interconnect 22 is stretched. The stretch range ofthe stretchable circuit assembly 20 is typically between 0% and about125% of the length of the stretchable interconnect 22.

One or more of the flexible circuits 23 can be replaced by strain reliefcircuits 25. The strain relief wires 25 prevent the stretchableinterconnect 22 from being over stretched, which protects the flexiblecircuits 23 from being damaged. Preferably, as shown in FIGS. 3A-4B, thetop and bottom flexible circuits 23 are replaced by strain reliefcircuits 25. However, no strain relief circuits 25 can be used; onestrain relief circuit 25, e.g., in the middle of the stretchableinterconnect 22, can be used; and more than two relief circuits 25,e.g., alternating with the flexible circuits 23, can be used.

The strain relief circuits 25 are typically made of flexible circuitsjust as the flexible circuits 23 but without any conductive lines.However, the strain relief circuits 25 can be made of any suitablematerial, including, for example, a metal or carbon fiber. The strainrelief circuits 25 preferably have the same shape as the flexiblecircuits 23 as shown in FIGS. 1A-2B; however, it also possible for thestrain relief circuits 25 to have a shape different from the flexiblecircuits 23.

Any suitable printed circuit board can be used for the printed circuitboards 21 a, 21 b. Although not shown in FIGS. 2A-4B, the printedcircuit boards 21 a, 21 b can include active and/or passive componentsfor processing and/or modifying the signals transmitted through thestretchable circuit assembly 20. Although not shown in FIGS. 3A-4B, theprinted circuit boards 21 a, 21 b preferably include an electricalconnector for connecting the stretchable circuit assembly 20 toelectrical devices with a corresponding electrical connector. Theprinted circuit boards 21 a, 21 b can be attached to an electronicdevice via surface mounted connectors such as board-to-board, clamping,pressure fittings, or spring pins, or can be embedded within a secondaryprinted circuit board, drilled, and copper plated to create a viainterconnect.

Method of Making Stretchable Circuit Assembly

FIGS. 5A-8B show a method of manufacturing a stretchable circuitassembly according to a third preferred embodiment of the presentinvention. Although the following discussion of the method ofmanufacturing a stretchable circuit assembly involves the use offlexible circuits, the discussion is equally applicable to manufacturinga stretchable circuit assembly using conductive wires instead offlexible circuits, except that conductive wires are used instead offlexible circuits.

FIGS. 5A and 5B show an injection mold 30 according to a third preferredembodiment of the present invention. FIG. 5A shows an empty injectionmold 30, and FIG. 5B shows an injection mold 30 with the printed circuitboards 31 a, 31 b and the flexible circuits 32 loaded within theinjection mold 30 but before any polymer is injected into the injectionmold 30. The injection mold 30 includes mold top 30 a, mold core 30 b,and alignment pins 30 c for aligning the mold top 30 a with the moldcore 30 b. The mold top 30 a and the mold core 30 b define an injectionarea 30 d into which polymer is injected. The injection mold 30 includesa seal 30 e for sealing the injection area 30 d when polymer is injectedinto the injection mold 30. The injection mold 30 includes a hole 30 fthrough which polymer is injected. Although only one hole 30 f is shownin FIGS. 5A and 5B, the injection mold 30 can have more than one hole.For example, the injection mold 30 could have a hole on each side of theinjection mold 30.

The steps of using the injection mold 30 to manufacture a stretchablecircuit assembly will now be discussed. A flexible circuit assembly 32′is manufactured as shown in FIG. 6. Any suitable method can be used tomanufacture the flexible circuit assembly 32′. As explained above withrespect to individual flexible circuits 23, the flexible circuitassembly 32′ includes a flexible plastic substrate with one or moreconductive lines for transmitting electronic signals. The flexiblecircuit assembly 32′ can also include passive and/or active components.The flexible circuit assembly 32′ is divided into individual flexiblecircuits 32 and formed such that the ends of the flexible circuits 32preferably have an L- or reverse L-shape. The ends of the flexiblecircuits 32 are preferably arranged such that the ends are perpendicularor substantially perpendicular to the portion of the flexible circuits32 between the ends.

FIG. 7A shows an empty mold core 30 b. The mold core 30 b includes afirst row of pins 30 b 1 and a second row of pins 30 b 2. The first 30 b1 and second 30 b 2 rows of pins are moveable with respect to each otheralong directions A, B that are anti-parallel to each other. The pins ofthe first 30 b 1 and second 30 b 2 rows of pins shown in FIGS. 7A-7C areshaped to form semi-circular portions in the flexible circuits 32.However, the pins could have different shapes to form different shapesin the flexible circuits 32, as explained above.

After the flexible circuits 32 are manufactured, the flexible circuits32 are loaded into the mold core 30 b such that the flexible circuits 32are arranged between the pins of the first 30 b 1 and second 30 b 2 rowsof pins as shown in FIG. 7B. It is possible to replace one or more ofthe flexible circuits with strain relief circuits. After the flexiblecircuits 32 are loaded in the mold core 30 b, the mold core 30 b isclosed by moving the first rows of pins 30 b 1 in direction B and bymoving the second rows of pins 30 b 2 in direction A as shown in FIG.7C. By closing the mold core 30 b, the flexible circuits 32 are shapedto have an oscillating shape with semi-circular portions connected bylinear portions, as discussed above. Preferably, the flexible circuits32 are inserted into holes in the printed circuit boards 31 a, 31 b andare attached to the printed circuit boards 31 a, 31 b before any polymeris injected into the mold core 30 b. As discussed above, the flexiblecircuits 32 are preferably soldered to the printed circuit boards 31 a,31 b; however, it is possible to use other suitable methods to attachthe flexible circuits 32 to the printed circuit boards 31 a, 31 b.

FIGS. 8A-8C are side views of the mold core 30 b according to a thirdpreferred embodiment of the present invention. The mold core 30 b shownin FIGS. 8A and 8B is simplified compared to the mold core 30 b shown inFIGS. 7A-7C in that the mold core 30 b shown in FIGS. 8A and 8B only hasfour rows of pins: two first rows of pins 30 b 1 and two second rows ofpins 30 b 2. The mold core 30 b can have any number of first rows ofpins 30 b 1 and of second rows of pins 30 b 2. In addition to the first30 b 1 and second 30 b 2 rows of pins, the mold core 30 b also includesa base 30 b 3 and retraction plate 30 b 4.

After the mold core 30 b is closed, the mold core 30 b is mated with themold top 30 a using alignment pins 30 c. For the sake of simplicity,FIGS. 8A-8C only show the mold core 30 b and the injection area 30 d.After the mold core 30 b and the mold top 30 a are mated, a first shotof polymer is injected into injection area 30 d of the injection mold 30as shown in FIG. 8A. The first shot of polymer is allowed to set. Afterthe first shot of polymer sets, as shown in FIG. 8B, the retractionplate 30 b 4 is moved in direction C so that the top of the pins of thefirst 30 b 1 and second 30 b 2 rows of pins are aligned with the topsurface of the base 30 b 3.

As shown in FIG. 8C, after the top of the pins of the first 30 b 1 andsecond 30 b 2 rows of pins are aligned with the top surface of the base30 b 3, the mold core 30, including the first 30 b 1 and second 30 b 2rows of pins, the base 30 b 3, and the retraction plate 30 b 4, is movedin direction D so that a gap is formed between the top surface of thebase 30 b 3 and the bottom surface of the set polymer from the firstshot of polymer in the injection area 30 d.

After the gap is formed between the top surface of the base 30 b 3 andthe bottom surface of the set polymer from the first shot of polymer, asecond shot of polymer is injected into the injection mold 30. Thesecond shot of polymer is allowed to set. After the second shot ofpolymer is set, the injection mold 30 is opened and the stretchablecircuit assembly is removed from the injection mold 30.

After the stretchable circuit assembly is removed from the injectionmold 30, the stretchable circuit assembly can be cut into discretecircuits, can have secondary components assembled or connected to it,can be tested, and can have bonding operations performed on it. Thebonding operations include, for example, bonding the printed circuitboards 31 a, 31 b of the stretchable circuit assembly to metalstiffeners, chassis, housings, or other flexible printed circuit/printedcircuit board assemblies.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications, andvariances that fall within the scope of the appended claims.

What is claimed is:
 1. A method of making a stretchable circuit assemblycomprising the steps of: providing flexible circuits, a first printedcircuit board, and a second printed circuit board; shaping the flexiblecircuits to have an oscillating configuration; and forming a stretchableinterconnect such that the flexible circuits are embedded within thestretchable interconnect; wherein the stretchable interconnect has athickness in a smallest dimension of the stretchable interconnect; eachof the flexible circuits has a thickness in a smallest dimension of theflexible circuits, has a width perpendicular to the thickness of theflexible circuits, is connected to the first and second printed circuitboards, and includes two or more conductive lines that are spaced fromone another along the width of the flexible circuits; the width of theflexible circuits is substantially the same as the thickness of thestretchable interconnect; and the flexible circuits are embedded in thestretchable interconnect such that the conductive lines are spaced apartfrom one another along the thickness direction of the stretchableinterconnect and such that the thickness of the stretchable interconnectis parallel or substantially parallel to the width of the flexiblecircuits.
 2. A method of making a stretchable circuit assembly accordingto claim 1, wherein the step of forming a stretchable interconnect isperformed by injection molding.
 3. A method of making a stretchablecircuit assembly according to claim 1, wherein the step of forming astretchable interconnect uses a polymer.
 4. A method of making astretchable circuit assembly according to claim 1, further comprisingthe step of attaching ends of the flexible circuits to the first printedcircuit board and the second printed circuit board.
 5. A method ofmaking a stretchable circuit assembly according to claim 1, wherein aportion of the first and second printed circuit boards are embedded inthe stretchable interconnect.